In recent years, circuit boards for mounting semiconductors are required to have the boards small-sized and to accomplish high density mounting and high performance. Further, it has been a problem how to dissipate the heat generated from e.g. semiconductor elements in a narrow space limited by small-sizing or high power of e.g. the semiconductor elements. Particularly, a metal base circuit board having a circuit formed by bonding a metal foil via an insulating layer on a metal plate has been commonly employed for the reason that it is excellent in heat dissipation performance mainly in the electric power source field or the automobile electric installation field.
However, in a metal base circuit board, the thickness of the base board is usually from 1.0 mm to 3.0 mm, whereby it is difficult to reduce the thickness, and the installation site is rather limited. Further, it has a structure formed via a thin insulating layer on a metal plate, whereby there has been a problem such that noises are likely to occur, or malfunction of a module is likely to occur.
In order to shield noises or to increase the heat dissipation performance, a metal base multilayer board is, for example, known wherein an upper layer circuit board having a circuit is laminated, via an adhesive agent, on is the entire surface or a part of the surface of a metal base circuit board (Patent Document 1).
In such a construction, an adhesive layer poor in the heat conductivity is present between the metal plate and the upper layer board, and in a case where a high power element is mounted on the upper layer circuit pattern, the heat dissipation performance tends to be inadequate, and the temperature of the element tends to rise, thus leading to a problem of malfunction.
In order to solve such a problem of heat dissipation performance, a metal base circuit board having a highly heat conductive insulating layer is known (Patent Document 2).
However, the metal plate was thick and could not be installed as attached along the shape of a case such as a case having a curved surface, and thus the heat dissipation performance of the insulating layer could not be sufficiently utilized. Besides, a large space was required for its installation, since it could not be installed in a bent form, and there was a problem such that the module could not be small-sized.
On the other hand, a metal base circuit board having an insulating layer made of e.g. an epoxy resin containing an inorganic filler formed on a metal plate, and having a circuit pattern formed thereon, is excellent in heat dissipation performance and electrical insulating performance and thus is used as a circuit board for an electronic equipment for e.g. automobiles and communication instruments having highly heat generating electronic components mounted (Patent Document 3).
If a metal base circuit board can be optionally bent, the limitation in the installation site which used to be limited to installation on a flat portion, will be relaxed, and it will be possible to adhere, bond or closely attach by e.g. a screw it to a side or bottom surface or to a stepped or curved surface of a case, whereby an electronic equipment having a highly heat generating electronic component mounted, can be small-sized. Further, if a metal base circuit board itself can be made thin, it can be inserted or fixed in a narrow space, whereby an electronic equipment having a highly heat generating electronic component mounted, can be made thin.
It has been proposed that by a method of heating a metal base circuit board at a high temperature of at least 120° C., i.e. by carrying out bending or drawing in such a state that the metal base circuit board is heated at a temperature higher by at least 10° C. than the glass transition temperature (Tg) of the insulating layer, the metal base circuit board having a non-flat portion is used also as a case or an electronic circuit package (Patent Document 4).
Further, a light-emitting diode (LED) light source unit using a light-emitting diode (LED) as a light source, has been used in various fields, but, for example, in a light source for backlight of a liquid crystal display device, it has been common to use a small size fluorescent tube so-called CFL (compact fluorescent lamp).
The light source of the above CFL (compact fluorescent lamp) adopts a construction such that Hg (mercury) is sealed in a discharge tube, and ultraviolet rays discharged from mercury excited by electrical discharge will impinge on the phosphor on the tube wall of CFL (compact fluorescent lamp) and will be converted to visible light. Therefore, recently, from the viewpoint of the environment protection, it has been desired to use a substitute light source not using hazardous mercury.
As a new light source, one using a light-emitting diode (hereinafter referred to simply as “LED”) has been proposed. With LED, the light has directional characteristics, and particularly with a surface mounting type on e.g. a flexible board, the light will be taken out in one direction, and as different from a structure using conventional CFL (compact fluorescent lamp), the light loss is small, and thus, it is used as a light source for backlight in a planar light source system (Patent Document 5).
The backlight using LED as a light source is now being widely used as backlight of a liquid crystal display device along with reduction of the price, improvement in the luminous efficiency and environmental regulations. At the same time, along with the higher brightness of a liquid crystal display device and enlargement of the display area, the increase in number of LED mounted on e.g. a flexible board and the trend for higher output are in progress in order to improve the luminescence.
However, the light source of LED has low luminance efficiency, and majority of the input power to illuminate LED will be discharged as heat. When an electric current is conducted, LED generates heat and becomes to have a high temperature due to the generated heat, and in an extreme case, LED will be destroyed. Also in a backlight using LED as a power source, such generated heat will be accumulated in LED and the board having LED mounted, and along with the increase of the temperature of LED, the luminance efficiency of LED itself tends to deteriorate. Besides, if it is attempted to increase the number of LED to be mounted or to increase the input power in order to make the backlight bright, the heat generation will further increase, and it is therefore important to remove such heat.
In order to reduce the heat accumulation in the LED-mounted board and to reduce the temperature rise of a LED chip, it has been proposed that a metal film having a LED chip mounted on a LED chip-mounted surface of a LED-mounted board, a metal drive wiring to supply a drive current to the LED chip and a metal film pattern intended for heat dissipation, are formed, a metal film for heat dissipation is formed on a surface facing the LED chip-mounted surface, and a metal through-hole is formed in a thickness direction of the LED chip-mounted board to connect the metal pattern on one main surface side to the metal film for heat dissipation on the other main surface side, so that heat generation from LED is dissipated through the metal through-hole to the rear side metal film (Patent Document 6).
However, in a case where the shape of LED to be mounted is small, the area of the mounted metal film will be limited, and the number of metal through-holes which can be formed immediately beneath LED will be limited, and in a case where a metal film pattern can not be formed on the mounted board from the restriction of the board area, there will be a problem such that the heat generated by LED can not efficiently be released to the rear side of the board.
Further, if a metal base circuit board using a metal base plate having a thickness of 2 mm is used instead of the flexible board, good heat dissipation can be obtained without necessity to provide metal through-holes, but there will be a problem that the thickness of the board tends to be thick, and it will be necessary to enlarge the size for punching out from e.g. an electrode and a circuit pattern over the flexible board, whereby the board area is obliged to be large. Further, it is not possible to optionally bend a portion other than the LED-mounted portion, whereby e.g. the position to form an input terminal will be restricted.
Besides, if the thickness of the metal base of the metal base circuit board is made thin to have such a structure that the size for punching out from e.g. an electrode and a circuit pattern is made small like a flexible board, even when the metal base circuit board is slightly bent, the insulating layer will have cracking and become useless. Likewise, there has been a problem that the LED-mounted portion can not be optionally bent.
Further, a metal base circuit board using a metal foil of from about 9 to 40 μm having a conductive circuit provided via an insulating layer containing a heat conductive filler and having heat dissipation performance and good bendability at room temperature, instead of a conventional polyimide type insulating layer, has been developed, since it can be used as being bent at room temperature and has bending processability.
However, if the conductive circuit was bent at an angle of 90° with a very small radius of curvature of at most 0.5 mm, the insulating layer at the bent portion was likely to have cracking and become useless. Therefore, if it was reinforced with a coverlay having an epoxy adhesive layer formed on a polyimide film, it was possible to prevent cracking in the insulating layer at the bent portion, but the bendability decreased, and there was a problem that it became difficult to bend it at an angle of at least 90° with a very small radius of curvature of at most 0.5 mm.
Further, in a case where a circuit board for mounting a semiconductor or a small size precision motor is, for example, mounted, there has been a problem that noises are likely to occur, or malfunction of a module is likely to occur.    Patent Document 1: JP-A-05-037169    Patent Document 2: JP-A-09-139580    Patent Document 3: JP-A-62-271442    Patent Document 4: JP-A-2001-160664    Patent Document 5: JP-A-2005-293925    Patent Document 6: JP-A-2005-283852